1. Elementary Students' Eyes Are Already Almost as Long, Front to Back, as Adult Eyes
The eye is shaped like a ball. Light entering from the front forms an image on the retina, which is on the inner surface at the very back. This front-to-back length is called the axial length. A study by Keio University examined about 1,400 elementary and junior high school students in Tokyo. The average axial length was 23.41 mm for elementary students and 24.73 mm for junior high students [1]. By elementary school, the average is already close to the adult average. Junior high students have eyes that are about 1.3 mm longer than that. Because this was a survey in Tokyo, it does not represent the whole country.
Most nearsightedness is "axial myopia," caused by the axial length growing longer than normal. It is said that the longer the axial length, the higher the frequency of future eye diseases [1]. Glasses correct how you see, but they do not change the axial length itself. What is happening to the back wall of an eyeball that has stretched front-to-back?
According to the summary of the Keio announcement, as the axial length increases, the risk of complications such as macular degeneration and optic nerve disorders rises [1].
2. How a Stretched Eyeball Affects the Layers of Its Back Wall
The back wall of the eyeball consists of layers: the retina, choroid, and sclera, from the inside out. When the eyeball stretches front-to-back, this wall can be stretched out. In explanations by Japanese ophthalmologists, the retina is sometimes pulled thin overall. In severe cases, this leads to a condition called "myopic chorioretinal atrophy," where the retina may stop working [2].
Photos of the inside of the eye (fundus photos) can also reveal deformation of the wall itself. This is called a "posterior staphyloma," where the back wall bulges backward like a dent [3]. The International Myopia Institute (IMI) defines "pathologic myopia" as a state where the axial length is excessively stretched due to myopia, causing structural changes in the fundus (posterior staphyloma, myopic maculopathy, optic neuropathy) that can lead to loss of corrected vision [3]. A report from the Japanese Ministry of Health, Labour and Welfare also states that vision loss from pathologic myopia is caused by retinal lesions in the macula or optic nerve lesions resulting from eyeball deformation [4]. The detailed mechanism of how the wall thins is still debated, so it is safer to read that it is thought to occur due to eyeball deformation.
Of these deformations, which one affects vision the most?
3. How New Blood Vessels in the Macula Lower Central Vision
The "macula" is the center of the retina, the area used for looking at the center of things. In Japanese reports, the formation of new blood vessels in the macula (myopic choroidal neovascularization) is cited as the most common cause of declining central vision in pathologic myopia [4]. New blood vessels (MNV) form in the macula of about 10% of patients with pathologic myopia, making it a major cause of central vision impairment. It is noted that these vessels are often smaller and less active than in conditions like age-related macular degeneration, making them easy to miss [5].
Treatment is also advancing. For myopic choroidal neovascularization, injections into the eye (anti-VEGF therapy) have shown effectiveness. For tractional maculopathy, which occurs when the retina is pulled, vitrectomy surgery is an option [3]. However, not everyone with strong myopia develops these conditions. Are "high myopia" and "pathologic myopia" the same?
4. Does Everyone with High Myopia Develop Eye Disease?
No, they are not the same. The IMI clearly distinguishes pathologic myopia from "high myopia" [3]. High myopia refers to having strong lens power in glasses. Pathologic myopia refers to whether there are structural changes in the fundus. Proportions change meaning depending on which group is counted. According to the IMI, pathologic myopia affects up to 3% of the world's population. It is less common in children and young generations, increasing with age and stronger lens power [3]. When looking only at people with high myopia, it is written that 50–70% have fundus changes. However, this is the proportion of people with fundus changes, not the proportion of people who lose their sight [3].
In the Hisayama Study, 2,164 people aged 40 or older (initially without myopic maculopathy) were followed for 5 years. During this time, about 1% developed new myopic maculopathy. Age and axial length were independent risk factors. The "odds ratio" (likelihood of occurrence) for every 1 mm increase in axial length was 2.94 [6]. Other papers reviewing complications of high myopia report that the likelihood of retinal detachment and glaucoma is higher than in people without myopia. It is said that risk increases even with low to moderate myopia [7].
These three numbers have different groups being counted: the total world population, people with high myopia, and residents aged 40 or older. It is safer not to compare them side by side. Age and axial length are independent risk factors, with an odds ratio of 2.94 per 1 mm of axial length (95% confidence interval 2.19–3.95) [6]. So, what can be done in the future?
5. Why Slowing Down Myopia Progression Matters
According to a presentation by Yokohama City University, it is predicted that by 2050, about half of the world's population will be nearsighted, and about 10% will have high myopia [9]. The IMI states that suppressing the progression of myopia is important for reducing the risk of pathologic myopia [3].
In Japan, low-concentration atropine eye drops (0.025%) were approved for "inhibiting myopia progression" on December 27, 2024. In a domestic trial targeting 299 children aged 5–15, the change in lens power after 24 months was -1.006 D for the drug and -1.643 D for the placebo (drops without active ingredients). The difference was 0.637 D [10]. This result means that lens power still progressed in children who used the drug. Rather than curing or stopping myopia, it slowed the progression. This is not an article recommending drug use. Whether to use it is a matter for consultation with an ophthalmologist. The relationship between myopia progression and outdoor play is summarized in a separate article on outdoor time and myopia. So, what should we look at, having only watched vision numbers until now?
6. Why Axial Length Matters Beyond Vision Test Numbers
In the Ministry of Education, Culture, Sports, Science and Technology’s 2025 school health statistics survey, the proportion of people with uncorrected vision below 1.0 was over 30% in elementary school, about 60% in junior high, and about 70% in high school [11]. However, having vision below 1.0 does not necessarily mean myopia. Vision, lens power, and axial length are separate numbers.
Let’s try this. Look at school health check results or eye examination records from when you got glasses with an adult. Look for "vision" and "lens power," and check if the "axial length" number is written. If it is not there, you might ask an ophthalmologist. The difference between 23.41 mm for elementary students and 24.73 mm for junior high students is 1.32 mm. The Hisayama Study has the number "odds ratio 2.94 per 1 mm of axial length," but the subjects were adults aged 40 or older. You must not apply this to calculate children’s growth differences [6]. Uncorrected vision below 1.0 is not necessarily caused only by myopia.
In a public relations magazine from Tokyo Medical and Dental University, Professor Kyoko Ohno-Matsui says there are many cases where high myopia progresses to pathologic myopia with complications [8]. The Keio University press release and the Ministry of Health, Labour and Welfare research report are in Japanese, so you can access the original text from the source list.